Some of fastener products include copper alloy fasteners in which a copper alloy containing zinc, such as brass, red brass and nickel silver (hereinafter also referred to as a “Cu—Zn based alloy”) is used for parts (for example, an element row as an engaging portion, a slider for controlling engagement and disengagement of the element row and opening and closing a fastener, and the like). Zinc is an alloy element that is conventionally added to the copper alloy fastener because zinc has an effect of increasing strength, hardness and uniform deformation of the alloy by a solid solute effect, and zinc is inexpensive as compared with copper and has thus good economical efficiency.
However, the zinc element present in copper may cause a problem of remarkably deteriorating corrosion resistance. The fastener parts produced by using a copper alloy with higher zinc content and particularly via cold working such as press molding have caused a problem of season cracking due to residual working strain. If the Zn content in the copper alloy is more than 10% by mass, the season cracking resistance will be rapidly deteriorated.
To improve the season cracking resistance of the Cu—Zn based alloy, it is considered that the proportion of zinc in the alloy is decreased to less than 10%. However, such an alloy with decreased zinc content is not desirable as a copper alloy for fasteners, because the alloy will not only increase material costs but also have insufficient strength. Therefore, Japanese Patent Application Public Disclosure (KOKAI) No. 2004-332014 A (Patent Document 1) proposes a process for producing a Cu—Zn alloy having improved season cracking resistance, characterized by subjecting a Cu—Zn based alloy containing at least Zn of more than 10%, which has been cold-worked, to a treatment for reducing tensile residual stress or for bringing about a state of compressive residual stress on the alloy surface. As specific methods of the above treatment, this document lists surface hardening methods such as shot peening, shot blasting, sandblasting and steel ball shot blasting.
Further, there are also following documents which disclose that a crystal structure of the Cu—Zn based alloy is formed as a mixed phase of an α phase having a face-centered cubic structure and a β phase having a body-centered cubic structure, and the ratio of the phases is controlled, thereby improving the season cracking resistance.
For the purpose of providing a copper-zinc alloy product having improved season cracking resistance and stress corrosion cracking resistance as well as cold workability and appropriate strength, WO 2014/004841 (Patent Document 2) discloses a copper-zinc alloy product composed of a copper-zinc alloy containing zinc of more than 35 wt % and 43 wt % or less and having a two-phase structure of an α phase and a β phase, wherein a ratio of the β phase in the copper-zinc alloy is controlled to be greater than 10% and less than 40%, and crystal grains of the α phase and the β phase are crushed into a flat shape by cold working so that the crystal grains are arranged in the form of a layer. This document also discloses that the flat β phase crystal grain is preferably layered in a crossing direction to a direction in which season cracking due to residual stress or cracking due to stress corrosion cracking is developed.
For the purpose of providing a copper alloy for fastening which has improved manufacturability, season cracking resistance and cold workability, WO 2014/024293 (Patent Document 3) discloses a copper alloy for fastening whose structure is comprised of a mixed phase of an α phase and a β phase, and wherein the copper alloy has a composition represented by a general formula: Cubal.ZnaMnb in which the symbols bal., a and b are expressed in % by mass, bal. denotes the balance, 34≤a≤40.5 and 0.1≤b≤6, and unavoidable impurities may be contained; and satisfying the following equations (1) and (2):b≥(−8a+300)/7 in which 34≤a≤37.5  (1)b≤(−5.5a+225.25)/5 in which 35.5≤a≤40.5  (2).
This document also discloses that in order to improve the season cracking resistance, the ratio (%) of the β phase in the crystal structure is preferably 0.1≤β≤22.
On the other hand, the copper alloy fastener members have been subjected to an element surface treatment with a rust prevention agent represented by benzotriazole-based compounds, in terms of prevention of discoloration. For example, Japanese Patent Application Public Disclosure (KOKAI) No. H08-24012 A (Patent Document 4) discloses a method for producing a slide fastener chain via polishing and rust prevention treatments, comprising a series of steps of degreasing and neutralizing a slider fastener chain having copper or copper-based alloy elements attached thereon, and then subjecting the slide fastener chain to a chemical polishing treatment by dipping the slide fastener chain in a chemical polishing solution, pickling it, and further subjecting it to a rust prevention treatment by immersing it in a rust prevention solution, and then washing the slide fastener chain with water, drying it and coating it with clear coating and drying it.